Ink-jet printing generally involves the controlled delivery of ink drops from an ink-jet print cartridge reservoir to a printing surface. One type of ink-jet printing, known as drop-on-demand printing, employs a print cartridge or pen that has a print head that is responsive to control signals for ejecting drops of ink from an associated ink reservoir.
One type of drop-on-demand print head uses a thermal bubble mechanism for ejecting drops. A thermal bubble type print head includes a thin-film resistor that is heated to cause sudden vaporization of a small portion of the ink. The rapid expansion of the ink vapor forces a small amount of ink through an associated one of a number of nozzles in the print head.
Conventional drop-on-demand print heads are effective for ejecting or "pumping" ink drops from a pen reservoir, but require mechanisms for preventing ink from leaking through the print head nozzles when the print head is inactive. Accordingly, drop-on-demand techniques require that the fluid in the ink reservoir must be stored in a manner that provides a slight back pressure at the print head to prevent ink leakage from the pen whenever the print head is inactive. As used herein, the term "back pressure" means the partial vacuum within the pen reservoir that resists the flow of ink through the print head. Back pressure is considered in the positive sense so that an increase in back pressure represents an increase in the partial vacuum. Accordingly, back pressure is measured in positive terms, such as water column height.
The back pressure at the print head must be at all times strong enough for preventing ink leakage. The back pressure, however, must not be so strong that the print head is unable to overcome the back pressure to eject ink drops. Moreover, the ink-jet pen must be designed to operate despite environmental changes that cause fluctuations in the back pressure.
A severe environmental change that affects reservoir back pressure occurs during air transport of an ink-jet pen. In this instance, ambient atmosphere pressure decreases as the aircraft gains altitude and is depressurized. As ambient air pressure decreases, a correspondingly greater amount of back pressure is needed to keep ink from leaking through the print head. Accordingly, the level of back pressure within the pen must be regulated during times of ambient pressure drop.
The back pressure within an ink-jet pen reservoir is also subjected to what may be termed "operational effects." One significant operational effect occurs as the print head is activated to eject ink drops. The consequent depletion of ink from the reservoir increases (makes more negative) the reservoir back pressure. Without regulation of this back pressure increase, the ink-jet pen will eventually fail because the print head will be unable to overcome the increased back pressure to eject ink drops.
Past efforts to regulate ink-jet reservoir back pressure in response to environmental changes and operational effects have included mechanisms that may be collectively referred to as accumulators.
Described in U.S. patent application Ser. No. 07/805,438, U.S. Pat. No. 5,409,134, which application is owned by the assignee of the present application, is a pressure-sensitive accumulator for ink-jet pens. The accumulator described in that application provides an accumulator working volume that is sufficient for operating the pen notwithstanding extreme environmental changes and operational effects on the back pressure within the reservoir. The accumulator moves to change the overall volume of the reservoir, thereby to regulate back pressure level changes so that the back pressure remains within an operating range that is suitable for preventing ink leakage while permitting the print head to continue ejecting ink drops.
For example, as the difference between ambient pressure and the back pressure within the pen decreases as a result of ambient air pressure drop, the accumulator moves to increase the reservoir volume, thereby to increase the back pressure to a level, within the range discussed above, that prevents ink leakage. Put another way, the increased volume attributable to accumulator movement prevents a decrease in the difference between ambient air pressure and back pressure that would otherwise occur if the reservoir were constrained to a fixed volume as ambient air pressure decreased.
The accumulator also moves to decrease the reservoir volume whenever environmental changes or operational effects (for example, ink depletion occurring during operation of the pen) cause an increase in the back pressure. The decreased reservoir volume attributable to accumulator movement reduces the back pressure to a level within the operating range, thereby permitting the print head to continue ejecting ink.
Accumulators are usually equipped with internal or external resilient mechanisms that continuously urge the accumulators toward a position for increasing the volume of the reservoir. The effect of the resilient mechanisms is to retain a sufficient minimum back pressure within the reservoir (to prevent ink leakage) even as the accumulator moves to increase or decrease the reservoir volume.
Even with a large-working-volume accumulator as just mentioned, there may be instances where the accumulator reaches its maximum working volume (for example, to reduce the back pressure within the reservoir as most of the ink is depleted during printing) while an appreciable amount of ink remains in the reservoir. Continued printing to remove this remaining amount of ink could increase the back pressure (which can no longer be regulated inasmuch as the accumulator has reached its maximum working volume) by a level outside of the operating range, which increase would cause the problem of print head failure owing to too high a back pressure level.
To avoid this problem, some ink-jet pens incorporate a "bubble generator." A bubble generator is an orifice formed in the ink reservoir to allow fluid communication between the interior of the reservoir and the ambient atmosphere. The orifice is sized such that the capillarity of the ink normally retains a small quantity of ink in the orifice as a liquid seal. The geometry of the orifice is such that when the back pressure approaches the limit of the operating range of the print head, the back pressure overcomes the capillarity of the ink and the liquid seal is broken. Ambient air then "bubbles" into the reservoir to reduce the back pressure so that the print head can continue to operate. Ideally, when the back pressure drops, ink from the reservoir reenters the orifice and reinstates the liquid seal.
In the past, ink-jet pens of the type just described were usually disposed of once the reservoir was depleted.